The invention described here is an autostereoscopic technique that allows to acquire and display still, film and television images in such a way that they appear as three dimensional for an observer. The observer(s) perceive the reproduced objects and scenes exactly if he would view these objects in reality. The corresponding still, film and television images may be generated by acquiring of natural scenes using a camera or they may be entirely computer-generated. In addition to a special optical device, some image processing is required in order to obtain suitable images for the 3-D display. This technique is suitable for reproduction of still images as well as for animated scenes and it guarantees a perfect gray and color fidelity. Finally, there are no principle restrictions for the size of the display.
The technique described here makes it possible to acquire and display images autostereoscopically and it is characterized by the following features:
1. Within each position of an defined viewing zone an observer perceives the displayed objects stereoscopically. Since stereoscopy is one of the strongest visual factors of perception, an observer has the impression of perceiving an object that is actually in front of his eyes.
2. When the position of an observer changes, there is a corresponding change in the perceived object perspective while the stereoscopic impression is maintained. Therefore, it is possible to xe2x80x9clook aroundxe2x80x9d an object and to perceive object details that were hidden from a previous position. This property is known to us from observing holograms, however up to now it has not been possible to implement it electronically.
People are used to this type of xe2x80x9cautostereoscopicxe2x80x9d viewing from the natural perception since, in general, the head or eye pupils are always in slight motion, thereby producing corresponding changes in the position of observation (i.e. the perspective). Pure stereoscopic image displays where always the same two perspectives are seen independently of the position of the observer (such as with the aid of polarization or shutter glasses) are perceived as artificial after prolonged viewing, which can lead to a feeling of nausea with many observers.
There is no lack of suggestions for autosteroscopic techniques, although we cannot acknowledge each and every one of them here. A systematic presentation of techniques for 3-D imaging is found in Reference 1 (Takanori Okoshi, Three-Dimensional Imaging Techniques, Academic Press, 1976). References 2/3/4/5/6 refer to patents and publications for autosteroscopic techniques that characterize, to our best knowledge, the present state of the technology. However, in one way or another, all of these techniques have some disadvantages that prevents their practical use in most applications.
One of the first to propose time-multiplexing of perspectives was Colander (Reference 2:Robert B. Collander, 3-D Television, Movies and Computer Graphics without Glasses, IEEE Trans, Consumer Electronics, volume CE-32 (1), February 1986). His system works in the projection mode and uses a special projection screen with an integrated Fresnel lens. One or several projectors are mounted below the screen and rotate in relation to it. Its mechanical design and the special characteristics of its projection screen restrict the potential use of his system.
In Reference 3 (Jesse B. Eichenlaub, Three Dimensional Imaging System, U.S. Pat. 4,367,486, January 1983) and Reference 4 (Jesse B. Eichenlaub, Autosteroscopic Display with Illuminating lines and a Light Valve, European Patent EP0316465A1, May 1989), Eichenlaub describes a technique where perspectives are multiplexed by means of a special arrangement of light sources and a light modulator in front of it. The special feature of this system is its compact design. However, the light modulators available today do not have the required bandwidth for the reproduction of a larger number of perspectives.
In Reference 5 (G. B. Kirby Meacham, Autosteroscopic Displaysxe2x80x94Past and Future, SPIE Volume 624, Advances in Display Technology VI, 1986) Meacham presented a follow-up development of Collender""s technique that also allows compact designs. The perspective information has to be separated by dark areas, which effectively limits the image resolution and also makes the design more complicated.
In Reference 6 (Adrian Robert Travis, Colour Autosteroscopic Display, International Publication Number WO 95/14353, May 1995), Travis describes a method that is very similar to that of Eichenlaub. However, the design is changed in such a way that it avoids the use of the spatial light modulator. However, this is only achieved by a complicated system of optical components.
Another technique that comes very close to the requirements of a perfect autostereoscopic display is described in Reference 7 (D. Just and H. Runge, Patent DE 4123895 A1. 1991). It is based upon using an optical aperture (in practical applications a stripe-like aperture segment is generally used) that moves in an appropriate distance in front of a monitor. The monitor is modified so that it works at a higher image refresh rate than it is necessary for a normal image and television display. The aperture motion may be a implemented by a genuine mechanical movement or by a pseudo-movement of an electro-optically controlled aperture. A non-mechanical aperture motion, i.e, requiring an electro-optical shutter, would be the preferred solution for any practical system implementation. There is a number of suitable shutter technologies available, the most suitable shutters are ferro-electric liquid crystals with regard to their switching periods and available sizes. Other liquid crystals such as the double-Pi cell could also be used if the image refresh rates are not too high. This is made possible by using the technique of the invention described here. Furthermore, electro-optical crystals such as KTP are suitable candidates to be used as shutters. These crystals can be grown today with side lengths of 0.50 m in very short time.
Regardless of the practical realization of the shutter, perspective images that were acquired from the geometric centers of the aperture positions, e.g. by means of a camera, are shown on the monitor synchronously with the movement of the aperture. If the aperture moves sufficiently fast over the screen, the movement is not longer resolved by an observer and the perspective images merge into a three-dimensional perception. In the following we will designate this technique as the single-aperture technique, because only a single aperture is opened at any moment. The single-aperture technique is specifically characterized by the following properties:
There are no limitations for the position of an observer as a result of restricted observation zones as one finds with almost all other autostereoscopic techniques. The observer can move completely freely to the side and/or backwards and forwards without losing the autostereoscopic impression. Image quality is only compromised in extreme positions of observation, which for all practical purposes are completely irrelevant. For example, an observer very far away or very close to the shutter or an observer in an extreme lateral position may experience some degradations. The property described already implies that there is no restriction for an unlimited number of observers. Each observer perceives the reproduced 3-D scene from a perspective that is correct for his or her position.
The observer(s) do(Es) not require any visual aids (such as polarization or shutter glasses) or any other aids that are often felt as restrictive (such as head-tracking devices). Rather, they are completely free and unobstructed.
The technique only requires a few components: a xe2x80x9cnormalxe2x80x9d monitor with a modified higher image refresh rate than is normally used and a moveable aperture that is in a suitable distance from the monitor. The aperture may be implemented mechanical or non-mechanical. In addition, there is of course the electronic control mechanism that is indispensable for its correct functioning. No further optical devices are needed and no precise alignments are required between the monitor and the aperture.
Although this technique is impressive because of its simplicity, there are some disadvantages which are too restrictive for most commercial applications:
1) On the one hand, the design described in Reference 7 requires a large volume. At best, when the optical path is suitably folded, one needs approximately the dimensions of a large-scale CRT.
2) There is a dependence between the width of the aperture and the number of the perspectives that can be reproduced. This makes the technique not suitable for a low number of perspectives, in particular it can not be adapted to the stereoscopic case where only two perspectives are used.
3) The technique has rather high light losses so that it is necessary to have a bright screen.
4) The required image refresh rate is very high, and therefore it can only be realized with few available display techniques.
5) The use of perspective images as they are normally employed with other autostereoscopic techniques (refer to Reference 2 for this), results in geometric distortions that can be disturbing when observing displayed scenes.
Especially points 3) and 4) make it difficult to practically implement the system. The image refresh rate required results can be calculated by the multiplication of the standard image refresh rate for a flicker-free image (typically 50 to 70 Hz) and the number of the required perspective views (typically one perspective per degree of viewing angle). A large viewing angle of typically 90 to 180 degrees requires therefore refresh rates of several thousand Hertz. Besides this, the image brightness is reduced in proportion to the number of perspectives. As impressive as a wide viewing angle range of such a system is, it is under most practical circumstances irrelevant because one usually utilizes a relatively limited latitude of movement in front of a workstation computer or a video screen, e.g. slight head and body movements or slight changes in the distance to the screen. Furthermore, by computer-aided manipulation it is possible to rotate a 3-D object and to observe it from all sides. The invention described here in all of its various designs takes advantage of this insight and avoids the disadvantages of the technique described in Reference 7. Apart from the improvements in the image refresh rates and picture brightness, it is possible to achieve significantly more compact designs all the way to flat screen monitors. Some of the designs are suitable for both a very small numbers of perspectives (all the way down to the stereoscopic case) and a large numbers of perspectives so that it can be adapted to various applications. It is further possible to achieve distortion-free displays by proper generation and/or processing of the perspective images. This geometrical processing of perspective images may also be applied with advantage to the technique described on Reference 7.